Redistribution is a process whereby a resin of lower molecular weight is prepared from a resin of higher molecular weight by a reactive extrusion process. This invention relates to a method of enhancing the catalyst efficiency during polycarbonate redistribution. More particularly, it relates to a method of improving the catalyst efficiency during redistribution by employing an extruder screw design that provides a melt seal between the extruder throat and the vacuum port. A melt seal exists when the free volume within the extruder is sufficiently filled with molten resin so as to prevent the passage of gases between sections of the extruder.
Polycarbonates are well known high performance engineering thermoplastics characterized by many advantageous physical properties, such as high optical clarity, toughness, dimensional stability and excellent impact strength over a wide temperature range.
Typically, in a manufacturing environment, different grades of polycarbonate, characterized primarily by different weight average molecular weights, are prepared in separate lots. To transition from one lot, or molecular weight grade, to another results in the production of a significant amount of material with variable molecular weight until the production line is completely purged of the prior grade. Transition grade materials must be inventoried and later matched and blended with other grades of polymer to obtain a grade with commercially acceptable properties. Often there are inconsistencies between the properties of transition grade batches, causing variation in matching and blending from batch to batch.
Recycling processes have become of increasing importance in the last few years; they are viewed as one way to minimize the volume of waste material that must be disposed of in landfills or the like. Therefore, it is desirable to provide processes for recycling scrap polymers, including polycarbonates.
In view of the above, it would be desirable to provide a practical method of reforming polycarbonates, including used polycarbonates and newly produced polycarbonate that have an initial weight average molecular weight, into a variety of commercial grade polycarbonate compositions characterized primarily by different molecular weights. Such a process would eliminate transition grade polycarbonate and the aforementioned problems associated with it, and would also permit recycle of scrap and post-consumer polycarbonate.
A method of redistributing polycarbonate has been disclosed in commonly owned U.S. Pat. No. 5,414,057. That method comprises the step of melt equilibrating an initial polycarbonate composition in a mixture that includes a catalytic amount of a polycarbonate redistribution catalyst under reaction conditions such that a redistributed polycarbonate composition is formed.
In a typical redistribution process, the catalyst comprises a tetraalkyl ammonium base. During the course of the process, the tetraalkyl ammonium base can decompose to form a trialkyl amine. If it is not removed, this trialkyl amine will be present in the final redistributed product, resulting in a degradation of the polycarbonate's material properties. The trialkyl amine can be removed by extruder devolatilization via one or more vacuum ports in the extruder.
It is desirable to enhance the catalyst efficiency in a redistribution process so that the quantity of catalyst is minimized while the weight reduction of the polycarbonate is maximized. At the same time, it is desirable to prevent the pulling of air into the extruder as a result of the vacuum port(s), as the introduction of air into the extruder will quench the redistribution reaction.